1. Field of the Invention
This invention relates to an electrophotographic apparatus for rendering an image visible with color particles of toner, etc., such as a printer, a facsimile, or a copier, and in particular to a developing method in a developing process of forming a toner image on the surface of a record material and an electrophotographic apparatus using the developing method.
2. Description of the Related Art
A developing method and a developing unit in a related art will be discussed. A recording apparatus using an electrophotographic technology involves a developing process of rendering an image visible on the surface of a record material with color particles and a fixing process of fixing the visualized color particle image onto the record material. Powder called toner dedicated to electrophotography is used as the color particles.
The full surface of a photosensitive body is once charged and subsequently is irradiated with light, whereby partial discharge is executed. Here, potential contrast of charge and discharge areas is formed on the surface of the photosensitive body and is called an electrostatic latent image.
In the developing process, first a developer is used to charge toner particles of color particles. The developer is mixed powder of toner and carrier beads of magnetic particles. It is sealed in the developing unit and is agitated therein. At the time, the toner is charged by friction with the carrier beads. The developer is transported by a magnet roller called a developing roller to a developing position opposed to the electrostatic latent image on the surface of the photosensitive body. At this time, a "magnetic brush" where the developer is arranged like a brush along a magnetic line of force is formed at the position opposed to the photosensitive body. Thus, the developing method using the method of conveying the developer to the electrostatic latent image on the photosensitive body by means of the magnet roller is called magnetic brush development.
On the other hand, a method called bias development is frequently used as a method of visualizing an electrostatic latent image. In the bias development, a bias voltage is applied to a developing roller and toner particles charged by the action of an electric field produced between the latent image potential formed on the surface of a photosensitive body and the developing roller are separated from the developer on the surface of the developing roller and are moved to the surface of the photosensitive body, making an image. The above-described charge potential or discharge potential may be used is the talent image potential (namely, potential of image formation portion of photosensitive body). Generally, a method of using the charge potential as the latent image potential is called a normal developing method and a method of using the discharge potential is called a reverse developing method. Of the charge potential and the discharge potential, the potential not used as the latent image potential is called background potential. The bias voltage of the developing roller is set to the middle between the charge potential and the discharge potential and the difference from the latent image potential is called the developing potential difference. Likewise, the difference from the background potential is called the background potential difference. Normally, the developing potential difference on which the developing performance itself depends is set larger than the background potential difference. If the developing potential difference is large, a formed electric field, called a developing electric field, becomes strong and thus the developing performance is enhanced, needless to say. Likewise, the developing electric field can also be strengthened by a method of narrowing the distance between the developing roller and a photosensitive body or a method of reducing the electric resistance of the developer; the developing performance can be enhanced.
The method of using magnetic brush development for transporting a developer and using bias development to visualize an electrostatic latent image is a widely generally used developing method. This developing method is called magnetic brush bias development throughout the specification. The relative move direction between a developing roller and a photosensitive body may be the same or may be opposite. One developing unit may use more than one developing roller. A developing unit may comprise a number of developing rollers rotating in the same direction or may comprise a number of developing rollers rotating in different directions. In this case, a developing unit is also known wherein two adjacent developing rollers are made different in rotation direction so that they are rotated toward a photosensitive body from the position opposed to the developing rollers for making a developer branch to the photosensitive body as if the developer were a fountain from the position opposed to the developing rollers. Such a developing unit is disclosed, for example, in JP-B-54-10869, etc. Such a developing unit is called a fountain-type developing unit throughout the specification.
As a modified example of the described bias developing method of electrophotography, for example, a developing method as seen from long ago in JP-A-48-37148, etc., is also proposed wherein potential of charge and discharge areas of a photosensitive body is divided into two parts to provide an intermediate potential area, a first developing unit for executing normal development is placed in the discharge area and develops first toner, and a second developing unit for executing reverse development is placed in the charge area and develops second toner, whereby two types of toners are developed in one charge step and light application step (exposure step). In this developing method, toner is not developed in an intermediate potential area (called intermediate potential) on a photosensitive body having a voltage value sandwiched between the bias voltage value of the first normal developing unit and that of the second reverse developing unit and a background portion is formed as an image, so that it is possible to form an image made of two types of toners consisting of the background part, the first image part, and the second image part. This developing method is called the potential division developing method throughout the specification. In the potential division development, normally two types of toners are used as separate colors for the purpose of providing a dichromatic image. If the reverse development is adopted as the first development and the normal development is adopted as the second development, the potential division development is also possible.
The magnetic brush bias development involves a problem of making the margins of an image hard to develop toward the rotation direction of a developing roll. This problem occurs because a magnetic brush scrubs the surface of a photosensitive body as a mechanical factor and the potential of the photosensitive body with which the magnetic brush comes in contact changes abruptly from the background potential of a non-image part to the developing potential of an image part and thus the electrical characteristic of the developer cannot follow the change. In the normal magnetic brush bias development, the developing performance is enhanced for preventing an evil effect caused by the problem from occurring. However, in the potential division development, a potential difference larger from the potential difference between the background potential and the developing potential in the normal magnetic brush bias development may occur. That is, if the background part of an image comes in contact with the first or second image part, the potential corresponding to the background potential is intermediate potential; this state is similar to the relationship between the background potential and the developing potential in the normal magnetic brush bias development. However, on the boundary where the first and second image parts come in contact with each other, when the first image part is developed, the potential of the second image part corresponds to the background potential; when the second image part is developed, the potential of the first image part corresponds to the background potential. At the time, the background potential difference becomes larger than the developing potential difference; at this time, a state different from the relationship between the background potential and the developing potential in the normal magnetic brush bias development occurs. Thus, to execute development after the magnetic brush passes through the boundary, an evil effect is larger than that in the normal magnetic brush bias development wherein the background potential difference is smaller than the developing potential difference, and a problem of an image loss with the boundary neighborhood of the image undeveloped and made void is involved. This problem can occur in either the first or second color in principle depending on the magnetic brush scrubbing direction; in fact, however, only the image loss of the first color is noticeable. The reason is that the toner image of the first color already developed exists before the magnetic brush of the second color passes through the boundary and that the toner of the first color causes the image part potential of the first color to approach the intermediate potential and thus the image part potential difference between the first and second colors lessens as compared with the case where toner does not exist.
The described potential division developing method in the related art is lacking in considering the image part potential difference between the first and second colors and the magnetic brush scrubbing direction and involves a problem of an image loss occurring on the boundary where the first and second color images are contiguous to each other.